In cognitive radio, which is a radio communication system in which parameters used for radio communication are changed adaptively according to a surrounding radio environment, the parameters are optimized according to the radio environment by recognizing the surrounding radio environment (detecting a radio signal). In particular, in a frequency band allocated to another radio communication system (hereinafter, referred to as primary system), if a cognitive radio system uses the frequency band in a shared manner as a secondary system, use efficiency of the frequency band improves.
When the secondary system uses the frequency band in a shared manner with the primary system, the secondary system tries not to affect existing services provided by the primary system. In order to avoid interference to the primary system, the secondary system needs to use a frequency band that is not being used by the primary system, or needs to perform such communication that produces an interference amount equal to or smaller than an amount allowed by the primary system. In other words, the secondary system needs to accurately identify a status of frequency band usage of the primary system before using the frequency band.
As a specific method of detecting whether or not a signal of the primary system exists in a frequency band used by the primary system (frequency band that the secondary system intends to use), there is known spectrum sensing, which is means used by a secondary system radio apparatus for detecting a radio signal in its surrounding. The spectrum sensing is broadly divided into the following methods. That is, a method using energy detection, in which a determination is made based on the amount of received signal power obtained through time averaging (energy detection), and a method in which a feature value contained in a transmitted signal of the primary system is used for detection (feature detection).
IEEE 802.22 is one example of the radio communication system in which detection of the primary system is performed through the above-mentioned spectrum sensing and the secondary system uses a frequency band that is not being used by the primary system. With IEEE 802.22, the standardization of wireless regional area network (WRAN) systems using the frequency band allocated to the U.S. TV broadcast has been discussed. According to IEEE 802.22, in a case where the received power of a signal compliant with Advanced Television Systems Committee (ATSC), which is a standard for the U.S. TV broadcast, is equal to or larger than −116 dBm, a misdetection rate and a false alarm rate are each defined to be set to 0.1 or smaller.
Here, the misdetection rate refers to a probability of determining that a searched frequency band is in an unoccupied state despite a fact that a signal of the primary system exists. The false alarm rate refers to a probability of determining that a signal of the primary system exists despite a fact that a searched frequency band is in an unoccupied state. The misdetection of a signal of the primary system leads to interference to the primary system, and the false alarm results in decreased frequency use efficiency.
FIG. 1 is a diagram illustrating, as an example, relation between the secondary system radio apparatus using the spectrum sensing and the primary system.
FIG. 1 illustrates a primary system radio apparatus 100 performing transmission, a primary system radio apparatus 110 performing reception, and a secondary system radio apparatus 200 that identifies a status of frequency band usage through the spectrum sensing. Further, a reference received power area 10 represents an area in which the misdetection rate and the false alarm rate of the secondary system radio apparatus 200 need to be controlled to predetermined values or smaller with regard to the detection of the primary system. Specifically, as illustrated in FIG. 1, in a case where the secondary system radio apparatus 200 is located within the reference received power area, the secondary system radio apparatus 200 is required to reduce the misdetection rate and the false alarm rate as much as possible, and also to control those rates to the predetermined values or smaller, by reliably detecting a signal transmitted from the primary system radio apparatus 100.
Further, also in cognitive radio communication systems other than the above-mentioned WRAN systems, similarly to the WRAN systems, it is necessary to protect the primary system against interference from the secondary system, and to keep high the frequency use efficiency of a used frequency band. Therefore, it is necessary for the secondary system to set the misdetection rate and the false alarm rate of the secondary system to the predetermined values or smaller with respect to a signal having a higher level than reference received power.
In order to set the misdetection rate and the false alarm rate to the predetermined values or smaller, various methods of detecting the primary system have been proposed. Among those methods, as a detection method in which a feature value contained in the transmitted signal of the primary system is used for detection, there are a method using cyclostationarity of a signal transmitted from the primary system, a method using cyclicity contained in the transmitted signal or a frame format, a method in which the secondary system radio apparatus prepares the same sequence as a pilot signal sequence of a received signal to examine correlation with the received signal, and the like.
For example, Patent Document 1 discloses a spectrum sensing method in which the chi-square test is performed using the cyclostationarity of a signal transmitted from the primary system. In this method, a cyclic autocorrelation value, which is a feature value reflecting the cyclostationarity, is generated and then compared with a threshold that is set in advance, to thereby determine whether or not a signal of the primary system exists. Further, the threshold is determined based on a chi-square distribution according to the false alarm rate set in the secondary system radio apparatus. A characteristic thereof is that the threshold can be determined only with the set value of the false alarm rate independently of noise power and interference power, and hence there is no need to estimate the noise power or the interference power. Further, by adjusting an averaging time required to generate the cyclic autocorrelation value, the misdetection rate can be set to the set value or smaller in an area having power equal to or larger than reference received power.
Other technologies are described in Patent Document 2, Patent Document 3, Non-patent Document 1, and the like. The cyclicity contained in the transmitted signal or the frame format, which is used in the spectrum sensing method of Patent Document 2, also exists in an OFDM signal using a cyclic prefix, and hence it is possible to detect the OFDM signal by using the method of Patent Document 2. Further, by using the same characteristic as the cyclicity of the cyclic prefix employed in Patent Document 2, as described in Patent Document 3, an application to blind estimation of an effective symbol length and a guard interval length, which are parameters of the OFDM signal, is also possible.
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2006-222665
Patent Document 2: US Unexamined Patent Application Publication (US-A1) No. 2007/0092045
Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 2007-082185
Non-patent Document 1: D. Cabric, S. M. Mishra, R. W. Brodersen, “Implementation issues in spectrum sensing for cognitive radios,” the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers (November, 2004)